If the pulsar is not strong enough for wideband ionospheric calibration,
we will use in-beam calibration. This requires preliminary VLA imaging
to identify possible in-beam calibrators (using A-array, 1.4 GHz data,
obtained from archives and ongoing), with follow-up
observations at the VLA (A-array, 8.4 GHz) and the VLBA (1.4 GHz) to
confirm compactness and suitability. Our past experience suggests that
about 1/3 of the investigated pulsars will be found to have suitable
in-beam sources within the 30 arcmin primary beam of the VLBA
telescopes at 1.4 GHz.
For cases where the pulsar is modestly strong (flux density >
10 mJy), the in-beam source can be as weak as 2 mJy, since we can
self-calibrate the pulsar and perform relative astrometry on the
in-beam source. For weaker targets, an in-beam source stronger than
about 8-10 mJy will be required to obtain differential phases and
delays with respect to the nodding calibrator.